A number of theories have been proposed regarding the mechanisms of systemic aging, and genetic, behavioral and environmental factors may all be involved. Evidence accumulated over the past decade has found measurable and successive age-dependent decline in stem cell activity from adulthood to old age to affect tissue homeostasis during natural aging. Identifying biomarkers indicative of stem cell aging and understanding mechanisms under which aged stem cells become functionally similar to young stem cells are important for monitoring aging and devising treatments for aging-associated imbalance in tissue homeostasis and tissue regeneration. Hematopoietic stem cells (HSCs) continuously provide mature blood cells during the lifespan of individuals in a process termed hematopoiesis. A progressive reduction in the immune response in the elderly and an increased incidence of myeloid malignancy, as well as anemia is thought to be linked to HSC aging. Molecular mechanism of HSC aging remain unclear, hindering rational approaches to identify useful biomarkers and to slow or reverse the decline of HSC function with age and to improve the quality of life upon aging. Identifying valid biomarkers of aging in hematopoiesis will assist n diagnosis and treatments for age- related blood conditions and diseases. Published work from our labs indicates that in mouse models, HSCs lose polarity upon aging, a phenomenon associated with stem cell intrinsic age-dependent elevation of an intracellular enzyme, Cdc42, and a canonical Wnt3A to non-canonical Wnt5A signaling switch. Our data further demonstrates that genetic and pharmacologic targeting of this Wnt5a-Cdc42 signaling axis functionally rejuvenates aged HSCs. Interestingly, published genome-wide association studies have found that Cdc42 expression in peripheral blood cells is closely linked to morbidity and mortality in humans, and our preliminary data demonstrate apolarity in human HSCs upon aging and a positive correlation between Cdc42 activity in peripheral blood cells and age of the blood donor, implying that Cdc42 might be a valid biomarker and/or valid target of aging in hematopoiesis. In the present studies, we propose integrated experimental and observational strategies to seek a causal relationship of our novel signaling switch in HSCs, Wnt5A-Cdc42, with aging phenotypes of HSCs at the single stem cell level, including myeloid-lymphoid lineage imbalances and the aged associated reduction of stem cell self-renewal and engraftment, using state-of-the-art mouse models. We will further examine the potential of targeting the Cdc42 signaling axis, key for HSC polarity regulation in aging, for rejuvenation of several HSC aging phenotypes. Our studies will establish a new mechanism of blood stem cell aging in animal models that could be translated to human aging, and may have values in other stem cell systems and tissue/organ regeneration.